Normally wood composites, particularly those used for industrial or construction grade products made with phenol formaldehyde resins as the adhesive have glue lines that are dark color and clearly visible after consolidation.
It is known to produce wood composite products having light colored or unnoticeable glue lines by utilizing special adhesives. Isocyanate adhesives, for example, are employed particularly when steam pressing and/or high moisture content wood are used. The isocyanate resin on curing tends to be light in color and therefore unnoticeable in resultant product, however, uncured isocyanate resins do have toxic implications during the manufacture and under certain consolidating conditions. Isocyanates normally do not present a serious health problem when properly handled or contained. Composites made using isocynates such as methylene di-phenylene isocyanate (MDI) when subjected to ultra-violet (UV) radiation, darken significantly resulting in the glue line becoming more visible.
In Europe it is relatively common practice to use melamine formaldehyde as a bonding resin for the manufacture of wood composite boards. These melamine resins produce satisfactory product but are more expensive than phenol formaldehyde resins and have limited approval for structural use in North America. The acid curable melamine or phenol melamine or phenol melamine urea resins are not compatible with the alkaline phenol formaldehyde resins and thus require that the plant or production line be substantially dedicated to the use of melamine formaldehyde resins or alternatively dedicated to the use of phenol formaldehyde resin or in some manner ensure that melamine adhesive does not contaminate the phenol formaldehyde or vice versa.
It is known that light colored phenolic resins may be produced using bisphenol-A formaldehyde resin. In a paper entitled Infra-red Spectra of Phenolic Resins by Nakamura, published in the Journal of Industrial Chemistry, Vol. 60, No. 6, 1957, pp. 785-789, the production of light colored alcohol stage B phenolic resins from bisphenol A (BPA) is discussed and various parameters examined for the production of colorless bisphenol A type resin. The description teaches the use of 1% potassium hydroxide as the catalyst and that curing at 140.degree. C. and above results in conversion of the resin to a dark yellow color. Thus, if used as an adhesive in conventional processes for producing lignocellulosic composite materials, wherein the temperature exceeds 140.degree. C., the resultant glue lines would be expected to be dark yellow as opposed to being light colored and nondistinguishable from the natural wood color.